refactor(modules): move base modules under modules

modules/guest.nix

@@ -0,0 +1,14 @@
+{ config, lib, ... }:
+let
+  cfg = config.modules.guest;
+in
+{
+  options.modules.guest.enable = lib.mkEnableOption "nested-guest base configuration";
+
+  config = lib.mkIf cfg.enable {
+    system.stateVersion = "26.05";
+
+    modules.ssh.enable = lib.mkDefault true;
+    modules.ssh.hostKeys.restore = lib.mkDefault false;
+  };
+}